The present invention relates to electrical indicators and particularly to an electrical indicator for use as part of an electrical tachometer.
Tachometers using so-called bi-torque or two coil ratio meters for indication usually remain pointed to the last speed reading at the time the instrument is deenergized. When these instruments are used in an automotive vehicle, the indication remains at the speed of the engine when the ignition is cut. To some users, this gives the impression that there is something wrong with the tachometer. To overcome this problem methods have been sought to return the pointer to zero after power is removed.
In British published patent application No. 2,102,580A, there is disclosed an electrical indicator having a relatively strong single or pair of return magnets that pull the rotor and therefore the pointer to the zero position. However, the strong zero return magnets, absent appropriate compensation, would greatly distort the indicator scale. To avoid this distortion, a compensating winding is added to neutralize the field from the zero return magnets whenever the indicator is engergized.
Based upon direct examination of indicators made by the applicants for the British patent and purportedly in accordance therewith, it has been determined that the strong zero return magnets require approximately 13 ampereturns to balance out the permanent field. Since the strength of the return magnets vary somewhat from specimen to specimen, each individual indicator must have the current through the bucking or compensating winding individually adjusted or calibrated necessitating extra labor in manufacturing.